
Both \(U(1)\)  groups are visables. It means that SM fermions carry both quantum numbers. This possibility usually arises in the context of grand unified theories (GUTs). Examples are the models of Babu et al. and that of Galison and Manohar [1]}. Models in which \(U(1)_1 = U(1)_y\)  and \(U(1)_2 = U(1)_{B-L}\)  are of this type.

There are fermions which carry only one of the \(U(1)\)  charges, others carry both charges and some with no charges. As an example of these sort of models we have del Aguila et al. in  [1]} .

One of the \(U(1)\)  factor is visible, the SM particles carry one of the \(U(1)\)  charge, for example \(U(1)_1\) , an the other, \(U(1)_2\)  is dark. Our model is of this type.

